Broadly speaking, you'll need to know two things: what you want and what a particular Web builder can give you.
First, assess your priorities. Do you want the equivalent of an online brochure? Do you have a catalog or product database that you'd like to put online? Are you planning to open
Next, think ahead a little bit. Do you have the knowledge to host your own Web server, or would you like someone else to worry about that? Can you maintain the site once it's built, or will you need help? Web developers offer a variety of long-term service and support options, including Web hosting and maintenance agreements. For a brief introduction to Web hosting, check out Web Hosting Services for Small Businesses.
Do your homework before you approach a Web development firm. Many Web sites include links to the companies that built them; if a particular site catches your attention, find out who created it. As a rule, you'll want to deal with firms that have a local office. Even though the Web is an international medium, you don't want to pursue a long-distance relationship with a Web builder.
Once you've selected a few candidates, check their work and their references carefully. Don't judge a Web builder just on the size or importance of its clients — decide whether a firm has experience creating the kind of site that you need.
For help finding a Web designer, be sure to check out Simple Steps to Finding a Web Designer and How to Choose a Web Design Company.